1. Field of the Invention
The present invention relates to a radio base station for performing radio communication with a mobile terminal, and particularly to a radio base station for performing so-called transmission power control of inserting a power control (TPC: Transmitter Power Control) command into transmission data so that reception quality of signals from a mobile terminal is constant, and also adjusting transmission power by a TPC command from the mobile terminal, and a control method thereof.
2. Description of the Related Art
The construction of a radio base station for performing transmission power control using a prior art will be described with reference to FIG. 8 by exemplifying a W-CDMA (Wide Band—Code Division Multiple Access) system. FIG. 8 is a block diagram showing the construction of a conventional radio base station.
As shown in FIG. 8, the conventional radio base station is equipped with base band transmitting/receiving parts 11, 12, 13, . . . for performing base band transmission/reception of each call under the control of a base station, and a channel (CH) multiplexing part 21 for multiplexing channels and outputting a base band (BB) transmission signal.
The base band transmitting/receiving part 11 takes charge of a channel zero (#CH0), the base band transmitting/receiving part 12 takes charge of a channel 1 (#CH1), and the base band transmitting/receiving part 13 takes charge of a channel 2 (#CH2).
Each base band transmitting part comprises an encoding part 101 for encoding transmission data, a frame generating part 102 for inserting a pilot symbol for synchronous detection and a TPC command into the encoded transmission data, a modulating part 103 for performing primary modulation such as QPSK (Quadrature Phase Shift Keying), 16 QAM (16-positions Quadrature Amplitude Modulation) or the like, a spreading part 104 for performing spreading modulation by using a spreading code of each call, a power control part 105 for determining a power value of each call, a power setting part 106 for multiplying the spread signal of each call by the power value determined in the power control part 105, de-spreading parts 107-1, 107-2 for performing de-spreading from a reception signal by using a reference code of each call and extracting the reception signal of the call, detecting parts 108-1, 108-2 for subjecting the extracted reception signal of each call to synchronous detection, an SIR measuring part 109 for measuring reception quality (SIR: Signal to Interference Ration: the ratio of desired wave power to interfering wave power) of the extracted reception signal of each call, an insertion TPC generating part 110 for determining an insertion TPC command from the measured SIR, a TPC judging part 111 for judging the TPC command out of the synchronously-detected reception signal, a data judging part 112 for judging transmission data out of the synchronously-detected reception signal, and a decoding part 113 for decoding the judged transmission data.
Furthermore, the CH multiplexing part 21 is a site for adding and multiplexing transmission-processed data of each call.
The operation of the conventional radio base station shown in FIG. 8, particularly, the operation associated with the transmission power control will be described.
First, the transmission data encoded in the encoding part 101 is input to the frame generating part 102. In the frame generating part 102, the synchronous detection pilot system and the TPC command are inserted into the transmission data.
As shown in FIG. 9, the pilot symbol and the TPC command are inserted at a period of one slot (slot) of the transmission signal. FIG. 9 is a diagram showing an example of the frame construction of a downlink (Downlink: DL) and an uplink (Uplink: UL). In FIG. 9, p represents a pilot symbol, d represents a data symbol and t represents a TPC command.
With respect to the pilot symbol, a unique symbol for which a slot number allocated on the basis of the system timing of the call concerned has been already known at both the base station and the terminal is inserted.
With respect to the TPC command, for example, SIR is measured from the received pilot symbol of a call A (UL) of FIG. 9, and the insertion TPC command is determined and generated in accordance with the measurement result in the insertion TPC generating part 110. In general, a reference SIR and the measured SIR of the call concerned are compared with each other, and the following commands are generated:
in case of measured SIR>reference SIR, DOWN command, or
in case of measured SIR<reference SIR, UP command.
The generated insertion TPC command is inserted into the next one slot of the transmission signal of the call A (DL) in the frame generating part 102, and the framed transmission data are modulated in the modulating part 103, spread in the spreading part 104, and input to the power setting part 106.
The received BB signal is de-spread in the de-spreading parts 107-1, 107-2 by the reference code of the call concerned, and subjected to the synchronous detection in the detection parts 108-1, 108-2. With respect to the TPC command, the judgment of the TPC command symbol is made in the TPC judging part 111, and with respect to the data symbol, the judgment of data is made in the data judging part 112.
The data judged in the data judging part 112 are decoded in the decoding part 113, whereby reception data can be obtained.
Furthermore, the TPC command judged in the TPC judging part 111 generally has “power UP”, “power DOWN” information, and on the basis of a judgment result in the TPC judging part 111, the power control part 105 determines power of +1 dB for the present power value in case of “power UP” and −1 dB in case of “power DOWN”.
The determined power value is reflected at a timing as shown in FIG. 9, multiplied by the transmission data in the power setting part 106 and then output. The transmission data which are bit-expanded by the power value of each call are multiplied in the CH multiplexing part 21 and output.
JP-A-2005-159495 “TRANSMISSION POWER CONTROL METHOD” (see Patent Document 1) is known as a prior art concerning transmission power control in DS-CDMA (Direct Sequence Code Division Multiple Access) assuming that so-called transmission power control of adjusting transmission power from a counter station so that reception quality from the counter station is constant is carried out at some transmission/reception station.    Patent Document 1: JP-A-2005-159496
The above operation is carried out for plural calls in the radio base station. In this case, there is the following problem.
The problem will be described with reference to FIGS. 10 and 11. FIG. 10 is a schematic diagram showing the frame construction when a frame boundary of each call has an offset to a system timing relatively, and FIG. 11 is a schematic diagram showing the frame construction when the frame boundary of each call is coincident with the system timing.
In the case shown in FIG. 10, the permissible power setting time and the insertion TPC generating time of each call likewise has an offset to the system timing, and in other words, it equivalently means that the processing load of each call which is effective to the permissible time is dispersed.
On the other hand, in the case of FIG. 11, the permissible power setting time and the insertion TPC generating time of each call are completely coincident, and thus this equivalently means that the effective processing time of each call to the permissible time is required to be short, that is, the load is concentrated.
When the system permits this condition, high-speed processing must be executed so as to adapt to the load concentration. For example, when it is assumed that the above processing is executed by using a signal processing processor, it is considered that a high-speed processor which can also adapt to the load concentration is used.
However, there is a problem that a higher speed processor is more expensive.
Furthermore, as the processing construction of the above processing, when the processing is executed under the condition that the processing amount per unit time is determined, for example, there are two calls during one symbol time, there occurs a problem that the permissible time can be kept for some call, but the permissible time cannot be kept for another call. When the permissible time cannot be kept, parasitic oscillation caused by so-called power control delay occurs, and the reception quality is deteriorated at both the base station side and the terminal side.
The insertion TPC processing of FIG. 8 is executed in the frame generating part 102, and the modulation processing in the modulating part 103, the spreading processing in the spreading part 104 and the power setting processing in the power setting part 106 are sequentially executed subsequently to the frame generating processing.
That is, with respect to the timing at which the TPC command is inserted, in consideration of the processing time of the modulation, the spreading, the power setting, etc., this processing is required to be executed much earlier. This effectively means that the permissible insertion TPC generating time of FIG. 9 is further shorter, and thus the above problem when the load is concentrated is estimated to be more serious.